In recent years, high-power lasers are being increasingly used in various industries. Optical components, such as lenses, windows, etc. are prone to damage when exposed to high-power, high-energy laser irradiation. All optical materials will ultimately damage at sufficiently high laser intensities through processes intrinsic to the optical material. Such intrinsic optical damage is the result of high-energy deposition through multi-photon ionization and is determined by the material's bulk electronic structure. Such damage normally occurs at intensities in excess of 200 GW/cm2. In practice, however even the highest quality optical components can damage at fluences well below their intrinsic damage threshold.
There are currently various methods in use to prevent and/or mitigate optical damage. One solution is to produce high quality optical materials that resist such optical damage. Even the highest quality bulk optical material, however, is not immune from optical damage at the surface of the material. One source of surface damage is caused by the absorption of sub-band-gap light by optical damage precursors. Optical damage precursors are defects that are extrinsic to the bulk optical material. Absorption of high fluence light by these precursors result in the explosive ejection of material from the surface, which leaves pits that may be a few microns to tens of microns in diameter. Surface fractures typically accompany these pits leading to further degradation of the optical material upon further irradiation by high-power or high-energy laser. Such damage becomes increasingly problematic as the operating wavelength becomes shorter, moving from red to ultraviolet. Similarly, in general it has been found that the higher the fluence and the shorter the pulse-length, the higher the damage intensity. Depending on the details of the optical finishing process, fused silica typically exhibits surface damage when exposed to a 3 ns pulse of 355 nm light at fluences between 1-30 J/cm2, which is considerably lower than the (<100 J/cm2) intrinsic damage threshold of a high quality bulk material. In addition, repeated and prolonged irradiation causes the damage to proliferate to unacceptable levels.
The density and nature of optical damage precursors is highly dependent on the finishing and handling process used during fabrication of the optical component. Damage precursors are primarily associated with photoactive impurities, surface fractures, and laser induced damage sites. Photoactive impurities may be introduced into the near surface region of the optical component during polishing. Surface fractures may include fractures introduced during the grinding process, as well as scratches or indentation fractures formed during the polishing, cleaning, handling, or use of the optical component. Optically induced surface damage sites may include fractured locations.
Many solutions have been proposed to increase the resistance of fused silica optical components that may enable the optical component to withstand high fluence irradiation particularly in the ultra-violet (UV) portion of the spectra. One solution is to remove the photoactive impurities and fractures, both of which can act as light absorbing damage precursors, during the optical component fabrication process. For example, U.S. Pat. No. 6,099,389 to Nichols et al. describes use of several conventional controlled grinding steps followed by conventional polishing. To prevent residual ceria in the polishing layer from acting as an optical damage precursor, Nichols describes the use of zirconia-based slurry, or the use of an etching step to remove residual ceria from the polishing layer. Another proposed solution is to use a specialized fabrication process. For example, U.S. Pat. No. 6,920,765 to Menapace et al. describes a fabrication process that includes conventional grinding followed by a conventional polishing step and a Magneto-rheological polishing step. The polishing layer is subsequently removed by etching. Yet another method for producing optics with reduced damage is described in U.S. Pat. No. 6,518,539 to Hackel. There repetitive scanning by a laser with increasing fluences is performed over the entire optic to reveal location of each surface defect. Once a defect location is revealed, the defect location is treated using global heating or etching of the optical component. In addition, U.S. Pat. No. 6,620,333 to Brusasco discloses a method where the optical component is first irradiated with laser to initiate damage and then a laser ablation process is used to treat the damaged area. This type of destructive mitigation method risks making the optical component more susceptible to future damage or may end up damaging a different aspect of the optical component.
All of the conventional techniques discussed above require a degree of preciseness and control that is difficult to achieve. In addition, some of the methods ignore one of the most prevalent precursors of optical damage: fracture surfaces associated with scratches (dynamic fractures) and “digs” (static fractures). Thus, there is a need in the art for improved processes for increasing the resistance of optical components to optical damage. In addition, more efficient means for mitigating previously initiated optical damage sites is needed.